1. Field of the Invention
The present invention relates generally to a system for varying the capacity of a heat pump to match building loads as outdoor temperature drops, and more particularly to a system which modulates heat pump capacity to reduce auxiliary energy usage by controlling the composition of a zeotropic refrigerant mixture.
2. Description of the Prior Art
The heating capacity of a single speed air-to-air heat pump is directly proportional to the outside (evaporator) temperature. As the outside temperature falls, the suction pressure and temperature also fall. This causes both the suction vapor specific volume and compression ratio to increase. System heating capacity is thereby reduced and specific compressor work input is increased. Building heat load, on the other hand, is directly proportional to the temperature difference between indoor and outdoor air. Consequently, building heat load increases as outside temperature falls.
Building heat load 10 and system capacity 12 are plotted as functions of outside temperature in FIG. 1. The outside temperature at which building load equals system capacity is referred to as a balance point 14. If outdoor temperature is below balance point 14, system capacity will be insufficient to satisfy the heating needs of the structure. In order to maintain indoor temperature, system capacity will have to be supplemented by an auxiliary energy source 16.
The auxiliary energy required for an entire heating season is the difference between seasonal building load and seasonal heat pump capacity output below balance point 14. Auxiliary energy is usually supplied by electric resistance heating, which has a Coefficient of Performance (COP) of one. Since the COP of a heat pump is usually greater than one, the Heating Seasonal Performance Factor (HSPF) may be increased by reducing the auxiliary heat 16 required. Although the increase in HSPF is usually modest (depending on the climatic region), the reduction in peak load to the electric utility may be significant. The benefits of a reduced peak energy demand may be magnified if the electric power plant uses a less efficient electric generation system to meet peak demand.
To reduce the amount of auxiliary heat 16 required, heat pump capacity must be increased to match building load as outside temperature falls. The only commercially available heat pumps capable of matching system capacity to building load have been those which vary the volumetric capacity of a compressor. This volumetric capacity variation may be accomplished in several ways; the most common methods are two-speed compressor motors and frequency inverters for variable speed compressor motors.
The system capacity may also be controlled by varying the composition of a zeotropic refrigerant mixture. Specifically, by controlling mixture composition, the thermodynamic properties of the refrigerant mixture may be altered to increase system capacity as outside temperature drops. Although shifting the composition of a zeotropic mixture may be thought of as a chemical analogue to a variable displacement compressor, the two methods are not mutually exclusive.
The effects of increasing heat pump capacity to match building load are shown for a single speed system capable of shifting the composition of a zeotropic refrigerant mixture in FIG. 2. In FIG. 2, it is assumed that the heat pump would operate on the original refrigerant mixture until outdoor temperature falls to the balance point. FIG. 2 shows that the result of increasing the capacity below the balance point will be a reduced auxiliary energy requirement and an increase in the HSPF.
The capacity increase which may be achieved by changing the composition of a zeotropic refrigerant mixture has been demonstrated by many investigators. However, none of these proposed systems have proven to be commercially viable because they have either failed to produce significant composition shifts or the hardware cost and control complexities have been prohibitive. It is the purpose of this invention to provide a significant composition shift while taking advantage of the natural tendencies of heat pump operation so that system control is not overly complicated.
The prior art discloses many different types of proposed methods and systems for controlling the capacity of a heat pump by changing the composition of a zeotropic mixture. All of these may be classified into the two generic categories: (1) those that use distillation columns and (2) those that use accumulators. Generally, the accumulator methods are simple to implement but require a large refrigerant charge. Additionally, the theoretical maximum refrigerant composition change is limited to the difference between the original charged composition and the vapor composition that is in thermal equilibrium with the original mixtures bubble point. It will be shown that for accumulator systems to achieve their theoretical maximum composition change they must store all but an infinitesimal portion of the total refrigerant charge. However, it is impractical to construct a system that stores such a large percentage of the total refrigerant charge.
Conversely, column methods may achieve much larger composition changes at the cost of increased system complexity in terms of both hardware and control. Many of these proposed systems have been so complex that they would never be cost effective enough to be applied in the competitive residential heat pump market.
As a typical example of the prior art in the systems employing accumulators consider the U.S. Pat. Nos. 4,179,898 (Vakil), 4,217,760 (Vakil et al.), 4,218,890 (Vakil et al.), and 4,283,919 (Vakil). These patents disclose systems which shift the composition of the zeotropic mixture by regulating the liquid level in the low pressure accumulator. Since these vessels have a minimum of heat transfer at the surface, the entire vessel will be in an approximate state of thermal equilibrium.
A Temperature-Composition diagram for a zeotropic mixture of R32/R134a is shown in FIG. 3. In FIG. 3, thermal equilibrium means that vapor leaving the accumulator and the liquid remaining must lie on a horizontal tie line. Additionally, species conservation dictates that one point on the tie line must be concurrent with the vertical line defined by the original charged composition. The two limiting tie lines dictated by species conservation are shown in FIG. 3. It is theoretically possible for the system to operate so that the equilibrium state in the low pressure accumulator is represented by any tie line that lies between these two tie lines. Maximum system capacity will occur when circulating fluid, represented by vapor leaving the accumulator, has the greatest composition of the more volatile component. This vapor composition is defined by the tie line that extends from the original composition bubble point.
Although it is theoretically possible for an accumulator storage system to operate so that the circulating composition is that of the vapor in equilibrium with the bubble point, it is not feasible in practice. Since the liquid remaining in the accumulator and the vapor leaving must lie on the same tie line, as the composition of the more volatile component increases in the vapor it also increases in the stored liquid. Consequently, as the maximum capacity composition is approached increasingly larger percentages of the refrigerant charge will have to be stored in the accumulator. It is impractical to store large percentages of the total refrigerant charge because the excess refrigerant will have to be accommodated in the cooling mode when the low pressure accumulator is empty. As in the case of the above patents proposed by Vakil, a high-side receiver must be added to accommodate this excess refrigerant. An additional problem of accumulator systems is that the total refrigerant charge will have to be very large. A large refrigerant charge is undesirable for economic and environmental reasons.
Turning now to the use of distillation columns in systems, several typical examples of these systems are discussed below. U.S. Pat. No. 4,781,348, by Fujiwara et al., discloses a specific configuration for a distillation column (referred to as a liquid-gas contactor) and a specific configuration for a refrigeration cycle to incorporate the liquid-gas contactor. The refrigeration cycle that incorporates this liquid-gas contactor disclosed in this patent: 1) is such that the distillation column must handle the entire system mass flow, instead of handing only the liquid phase of the evaporator overfeed refrigerant; and 2) the liquid-gas contactor is located between two partial expansions that comprise the total expansion pressure drop of the system. This linkage between liquid-gas contactor and the expansion process complicates the control of the system. Finally, as the Fujiwara et al. system is configured, the system requires a cooler to liquify the vapor leaving the top of the liquid-gas contactor. In practice, the rate of heat removal and temperature level in the cooler is difficult to control and thus generates additional control problems.
U.S. Pat. No. 5,012,651, by Nakatani et al., discloses a heat pump apparatus which incorporates a fractioning/separating device (distillation column) with a reservoir and heat source. Because of the location of the fractioning/separating device, this system requires more than just a heater to control the operation of the fractionator. In all of the various embodiments shown in FIGS. 1 through 9, a valve is required to meter the flow of refrigerant from the reservoir to the heat source coil. Additionally, various complex variations of the Nakatani et al. system are necessary to allow the fractioning device to be functional in both the heating and cooling modes.
U.S. Pat. No. 4,840,042 (by Ikoma et al.); and Japanese Patents 2-64,364 (by Ikoma); 2-64,367 (by Ikoma); 2-64,368 (by Nakatani); 2-64,369 (by Ikoma); and 2-64,370 (by Ikoma) are all essentially the same as U.S. Pat. No. 5,012,651, described above, except for the introduction of the suction ejector to remove vapor from the top of the distillation column. Essentially, they all incorporate a distillation or fractionating device into the heat pump by merging an entirely separate ancillary system.
Soviet Union Patents 907,360 and 846,937, while related to refrigerating systems, do not describe vapor compression refrigeration systems as they contain no compressor. The pressure difference is established by liquid pumps and they contain no distillation column or fractionating device of any kind. The refrigerant composition differences are established by separation of partially evaporated refrigerant streams.
U.S. Pat. No. 4,722,195, by Suzuki et at., discloses a heat pump system equipped with a distillation column to vary the composition of a zeotropic refrigerant mixture in both the heating and cooling modes. This patent is essentially the same as U.S. Pat. No. 4,781,738, described above. The only difference is that the distillation column, with its two partial expansions, is now placed in a refrigerant flow circuit parallel to the main expansion process. This modification enables the system to theoretically function in both the heating and cooling modes.
U.S. Pat. No. 4,580,415, by Sakuma et al., discloses a refrigeration system designed to produce a rapid freezing mode for temporary ultra-low temperature operation. It should be appreciated that this system is not intended for a residential heat pumping application. Additionally, the system stores the low boiling point refrigerant R13 in the vapor phase on the high pressure side of the system.
Other patents of interest include: U.S. Pat. Nos. 2,938,362, by Schwind; 2,867,094, by Alpaus; 2,951,350, by Etherington et al.; 4,987,751, by Lewen; 2,841,965 by Etherington; and 2,951,349, by Etherington.
In all of the prior art, discussed above, the distillation of the zeotropic refrigerant mixture is accomplished in a manner that requires significant changes in the basic refrigeration cycle and hardware. Applicant's proposed system does not require any changes in the basic refrigeration cycle and adds no additional moving parts or valves. Consequently, the system hardware and control methods remain in their original simple form.